The present invention relates to an image forming apparatus for copying or printing an image by an electrophotographic process and an image forming unit incorporated therein and, more particularly, to an electrophotographic process control device for controlling each part of an electrophotographic process.
A copier, laser printer, LED (Light Emitting Diode) printer, facsimile transceiver or similar image forming apparatus of the type copying or printing an image by an electrophotographic process including charging, exposure, development, image transfer and fixation is conventional. The prerequisite with this type of apparatus is to control each part of the electrophotograhic process in an optimum way for forming an attractive image. Typical of conventional control methods is one using look-up tables. Specifically, this kind of control method measures the surface potential of a photoconductive drum and an amount of toner deposition by a surface potential meter and a photosensor, respectively. Look-up tables list optimum manipulation amount data relating to some different states of the apparatus which were determined by, for example, experiments. To control the electrophotographic process, the look-up tables are referenced to select manipulation data, i.e., a manipulation amount of toner supply and that of a bias voltage matching the above-mentioned measured values. Another conventional method changes the manipulation amounts of various sections inside of the apparatus and finds optimum operation amounts by conventional PID (Proportional, Integral and Differential) or similar control while feeding back the outputs of sensors disposed in the apparatus. Still another conventional control method uses a computing device having a fuzzy inference capability and manipulates each subject of control by total decision on the complicated tangle of numerous state parameters.
However, the conventional electrophotographic process control methods discussed above have some problems left unsolved, as follows. First, with the look-up table scheme, it is extremely difficult to determine an optimum manipulation amount for all the possible states of the apparatus. Specifically, conductive experiments by assuming all the possible states of the apparatus, including environmental condition, and holding tables listing all the resulting operation amount data in the apparatus is not practical. To cover all the expected states of the apparatus, a prohibitive amount of experiments and a prohibitive amount of data are needed. In light of this, it has been customary to list only acceptable manipulation amounts relating to typical states of the apparatus. This prevents optimum control from being effected in all the states of the apparatus. Moreover, the manipulation amounts have to be provided with some margins (errors) to cope with the scattering among machines (image forming units) ascribable to the production line, making it extremely difficult to effect optimum control in various states.
On the other hand, parameters representative of the state of the apparatus include, for example, the surface potential of a photoconductive drum and the amount of toner deposition on the drum which should not be frequently measured. In practice, therefore, the repetitive feedback scheme using PID control, for example, is difficult to execute. Specifically, it is a common practice with this kind of scheme to measure the surface potential of a photoconductive element and the amount of toner deposition and to change the voltage, current or similar manipulation value to be applied to a charger or a light source by, for example, PID control while feeding back the measured values. Such a procedure is repeated until the surface potential and the amount of toner deposition converge to respective target values. However, the problem is that the surface potential and the amount of toner deposition cannot be frequently measured. For example, assume a latent image is electrostatically formed on a photoconductive drum. To measure the surface potentials of the light (charged) and dark (exposed) portions of such a latent image, a reference latent image pattern for measurement is formed on the drum, and then the surface potential is measured in each of the charged and exposed portions (and some halftone portions, if necessary). Hence, the latent image pattern is developed without exception due to the inherent process, i.e., a toner is deposited on the drum. Should such a toner image have an excessively high density, it would critically increase the load on a cleaning unit and, in the worst case, make cleaning impossible unless the toner image is transferred to a paper sheet or similar recording medium. Transferring the reference toner image to a paper sheet would invite not only the waste of paper sheets but also the increase in the number of processing steps. Therefore, to protect the cleaning unit, the reference toner image should have a relatively low density or should be formed as scarcely as possible. In addition, the reference toner pattern aggravates toner consumption.
The amount of toner deposition on the photoconductive drum, too, cannot be measured unless the above-stated reference toner pattern is formed on the drum, again resulting in the cleaning problem. The allowable frequency of measurement is, therefore, limited. Moreover, the sequence of charging, exposing, developing and cleaning steps indispensable for the measurement is time-consuming. It follows that the repetitive measurement degrades the performance, i.e., copying or printing speed of the apparatus itself while increasing the interval between the first copy and the print output.
Since the parameters for determining manipulation amounts depend on one another in a complicated way, it is difficult to converge the control with the PID algorithm or similar conventional relatively simple algorithm.
The limitation on the measurement of the surface potential discussed above is, of course, true with the conventional fuzzy computation scheme also, since such control is not practicable unless the surface potential is known.